As an emission type electronic displaying device, an electroluminescent display (ELD) is known. Elements constituting the ELD include an inorganic electroluminescent element and an organic electroluminescent element (hereinafter also referred to as an organic EL element). Inorganic electroluminescent elements have been used for a plane light source, however, a high voltage alternating current has been required to drive the element.
An organic EL element has a structure in which a light emitting layer containing a light emitting compound is arranged between a cathode and an anode, and an electron and an electron hole are injected into the light emitting layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light by applying a relatively low voltage of several volts to several tens of volts. The element exhibits a wide viewing angle and a high visuality since the element is self-luminous. Further, the element is a thin, completely solid element, and therefore, the element is noted from the viewpoint of space saving and portability.
A practical organic EL element is required to emit light of high luminance with high efficiency at a lower power. For example, disclosed are: an element exhibiting higher luminance of emitting light with longer life in which stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives doped with a slight amount of a fluorescent compound are employed (for example, see Patent Document 1); an element which has an organic light emitting layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (for example, see Patent Document 2); and an element which has an organic light emitting layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (for example, see Patent Document 3).
When light emitted through excited singlet state is used in the organic EL elements disclosed in the above Patent Documents, the upper limit of the external quantum efficiency (ηext) is considered to be at most 5%, because the generation probability of excited species capable of emitting light is 25%, since the generation ratio of singlet excited species to triplet excited species is 1:3, and further, external light emission efficiency is 20%.
Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Princeton University (for example, see Non-Patent Document 1), studies on materials emitting phosphorescence at room temperature have been actively carried out (for example, see Non-Patent Document 2 and Patent Document 4). As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits almost the same performance as a cold cathode tube, and can be applied to illumination.
In “The 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu)”, papers on phosphorescent compounds have been reported. For example, Ikai et al. have reported a phosphorescent compound in which a hole transporting material had been used as a host material. M. E. Thompson et al. have reported a phosphorescent compound of which the host material has been doped with a novel iridium complex compound. Tsutsui et al. have reported an organic EL element exhibiting high luminance by employing a hole blocking layer (an exciton blocking layer). Other examples of a hole blocking layer include: a kind of an aluminum complex reported by Pioneer Corp., and a fluorine substituted compound as a hole blocking layer (an exciton blocking layer) exhibiting high luminance, reported by Ikai et al. in Appl. Phys. Lett., 70, 156 (2001).
Patent Document 1: Japanese Pat. No. 3093796
Patent Document 2: Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 63-264692
Patent Document 3: JP-A No. 3-255190
Patent Document 4: JP-A No. 2000-21572
Non-Patent Document 1: M. A. Baldo et al., Nature, 395, 151-154 (1998)
Non-Patent Document 2: M. A. Baldo et al., Nature, 403(17), 750-753 (2000)
However, in general, a phosphorescent element tends to exhibit a shorter life time compared to a fluorescent element, and an organic EL element employing a phosphorescent compound as an emitting material is not fully durable when it is continuously driven.
This may be due to the deterioration of a blocking layer provided to attain high luminance and a high emission efficiency.
Namely, electrons or holes drifted from the light emitting layer cannot be fully blocked by the blocking layer and some of the electrons or holes migrate into the blocking layer, whereby the blocking layer is deteriorated and the life of the element is shortened. Further, the excitation life of a phosphorescent emission dopant is rather long, accordingly, when the concentration of excitons is high, a triplet exciton collides with another triplet exciton in the vicinity to cause T-T annihilation, whereby the emission efficiency is reduced.
The present invention was achieved under the above situations. An object of the present invention is to provide an organic electroluminescent element exhibiting long life and a high emission efficiency and, and to also provide an illumination and a display employing the organic electroluminescent element.